Most, if not all, operating system programs for use with digital data processors require a reserved area of system read/write memory for the storage of operating system-related data such as device tables, interrupt vectors and so on. In many systems this reserved area of memory is predefined to be a "lowest" page of physical memory, often referred to as "page zero". Page zero will extend across a physical address range of zero to some larger address such as 1023 or 2047. The use of page zero for such operating system functions is advantageous if for no other reason than many data processors have a special form of addressing for page zero that includes a shorter instruction length resulting in a faster instruction fetch and execution.
A problem is created for those types of systems characterized as a multi-processor system wherein a plurality a data processors, each of which executes an associated controlling program such as an operating system, are coupled to a common system memory. Each of the data processors requires access to an associated page zero within the common memory. However, in that each of the data processors may be operating independently of the other data processors the processors cannot be allowed to "share" the page zero memory area. For example, if one of the data processors were to modify the page zero data the modified data may be inappropriate for use by the other data processors.
It is therefore an object of the invention to provide an apparatus for use in a multi-processor data processing system for enabling each of the processors to access an associated predefined region of system memory.
It is another object of the invention to provide an apparatus for use in a multi-processor data processing system for providing each of the processors an associated "page zero" within system read/write memory to enable concurrent execution of programs without conflict by the data processors.
It is a further object of the invention to provide a data processing system having n data processors each running a program requiring a predetermined region of system read/write memory and to also provide n regions of system memory each of which is mapped to appear, to an associated processor, as the predetermined region of system memory.
It is a still further object of the invention to provide a data processing system having n data processors each running a program requiring a page zero region of system read/write memory and to also provide n effective page zero regions of memory to enable concurrent program execution by the data processors.